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What your cholesterol number really says

Half of all heart attacks come with no warning at all, making diagnostic tests all the more important.

STORY HIGHLIGHTS

"The cholesterol number is essentially worthless," says Dr. Arthur Agatston

Average LDL of people hospitalized for heart disease was 105 -- "nearly optimal"

Younger women may be particularly jeopardized by a heavy reliance on cholesterol

Editor's note: Dr. Sanjay Gupta explores the signs, tests and lifestyle changes that could make cardiac problems a thing of the past on "The Last Heart Attack" at 8 p.m. ET Sunday on CNN.

(CNN) -- Learning your cholesterol numbers -- the good, the bad and the total -- is a well-established part of the annual physical exam.

A high cholesterol number results in angst and possible treatment. A low number brings relief. But are we putting too much stock in cholesterol as a sign of our heart attack risk?

"The cholesterol number is essentially worthless," says Dr. Arthur Agatston. The Miami cardiologist and South Beach diet books author says the cholesterol of people who have heart attacks and those who don't are almost identical.

"The overlap is troubling," Dr. Kwame Akosah at the University of Virginia agrees. "As an isolated predictor, it falls short."

As a sign of the weakness in cholesterol numbers alone, one large study found the average LDL cholesterol of people hospitalized for heart disease was 105, which is considered "near optimal."

The study, published in the American Heart Journal in 2009, found almost half of the hospital admissions had LDL cholesterol levels below 100, traditionally considered "low risk."

Another study found that only half of heart attacks occurred in people with high cholesterol (at or above 240), while a fifth of the heart attacks struck people whose cholesterol levels (below 200) deemed them safe based on long-held guidelines.

Agatston says the cholesterol numbers fall short because they measure cholesterol in your blood. They don't tell you the amount of LDL, or bad cholesterol, building up as plaque in the blood vessel walls. Plaque is what causes heart attacks.

As a result, studies show some people who think they're high risk based on cholesterol numbers are not, while others who think they are in the clear are developing dangerous plaques.

Your body needs cholesterol. The waxy substance is in the lining of every cell in your body. You also use cholesterol to make hormones, vitamin D and other substances.

Cholesterol circulates in the blood stream with the help of lipoproteins. The low-density lipoprotein, or LDL, carries cholesterol away from the liver. The high-density lipoprotein, or HDL, scavenges excess cholesterol and brings it back to the liver.

Your body produces cholesterol in the liver. You can also get cholesterol in fatty foods. When the LDL in the blood outstrips the HDL's ability to scavenge it, plaque can accumulate. The buildup of plaque is a lifelong process, beginning in childhood, but everyone is different.

Your genes play a role in whether you are prone to accumulating LDL as plaque. High blood pressure also makes you more prone to plaque buildup. If you are obese or diabetic or a smoker or suffer from a chronic inflammatory condition like lupus, you are also more likely to accumulate plaque than someone with the same LDL cholesterol number.

Akosah says he uses all these factors to determine a patient's "global risk" and tailors the treatment accordingly.

Also, there are different LDL particle sizes. Smaller particles tend to penetrate the vessel wall more easily than larger LDL particles.

Most heart attacks are not caused by the slow narrowing of blood vessels but by a rupture of a blister or bubble of plaque in an artery that is less than 50 percent blocked. Half of all heart attacks come with no warning at all, making diagnostic tests all the more important.

Younger women may be particularly jeopardized by a heavy reliance on cholesterol numbers. One study that looked at heart attacks in women under 65 found none had been deemed "high risk" for a heart attack using the traditional Framingham Risk Score, which looked at age, smoking, blood pressure, total cholesterol and HDL.

A class of drugs called statins lowers LDL cholesterol. Exercise and a low-fat diet rich in fruits, vegetables, whole grains and legumes can also lower LDL and raise HDL. Smokers who quit also improve their cholesterol numbers.

"High cholesterol is not a diagnosis. It's a symptom. It's like a fever. The first step is to figure out what's going on," says Dr. James A. Underberg, a professor at New York University School of Medicine who specializes in preventive cardiovascular medicine.